The present invention relates generally to media handling in printers. More particularly, the invention relates to a system for use in determining media type and selectively supporting printed media during printing so as to optimize printhead-to-media spacing and to reduce media skew in printers.
Although the invention has broad utility, it has proven particularly well-suited for use in inkjet printers which employ pivotal wings to support media above an output tray while ink on a preceding media sheet is allowed time to dry. Such printers, it will be appreciated, also may employ mechanisms which aid in support of an expelled media sheet by establishing a sheet-stiffening bow in the expelled media sheet. The present invention is described in the context of such an inkjet printer below.
In a conventional ink-jet printer, media is directed through a print cycle which includes picking up a media sheet from an input tray, feeding it through the printer""s print zone, and expelling it through an output port. Typically, expelled media is supported momentarily, before falling to an output tray where consecutive sheets pile, one on top of the other, to form an output stack. This momentary delay is useful inasmuch as it provides the previously printed sheet with time to dry before placing the present sheet on the output stack. Because inkjet printers print using wet ink, the aforementioned methodology is particularly useful in inkjet printers.
Printer manufacturers have employed a variety of mechanisms to accomplish this momentary delay in stacking printed media sheets. One such mechanism embodies a passive drop scheme, wherein a media sheet emerging from the printer""s output port is guided along rails which suspend the sheet above the output tray. At the completion of printing, the sheet simply drops of its own weight into the output tray, the previously printed sheet having had an opportunity to dry during printing of the present sheet. It will be appreciated, however, that passive drop schemes are not always reliable due to variations in media characteristics (e.g., media thickness), and/or due to a phenomenon known as cockling. As a result, sheets do not always drop into the output tray after printing. Instead, printed sheets may be pushed forward and out of the printer by the following printed sheet. This cockling effect may become more pronounced with environmental extremes and/or with large amounts of ink on a sheet.
Another solution involves the use of an active drop mechanism wherein a printed sheet is guided along a pair of movable wings which temporarily support the printed sheet above the output tray. Once printing is complete, the wings retract, often pivotally, allowing the sheet to fall into the output tray below. One such system is described in U.S. Pat. No. 5,324,020 to Rasmussen et al., the subject matter of which is incorporated herein by this reference. Although generally effective, active wing arrangements may not be suitable for a wide range of media widths, and may present some problems due to sheet sail, a phenomenon which can result in sheets sailing out of the printer upon retraction of the wings.
In order to address these issues, the printer may be provided with a system whereby a printed sheet is temporarily supported above the output stack in a sheet-stiffening bow. This may be accomplished using a guide mechanism with an elongate channel which receives a predetermined side edge of an expelled sheet. One such system is described in U.S. Pat. No. 5,603,493 to Kelly, the subject matter of which is incorporated herein by this reference. Similarly, the printer may employ pivotal wings as described above, but with an opposing pinch member configured to impart the aforementioned sheet-stiffening bow to the printed sheet.
Although such systems work well for most types of sheet media, difficulties still may arise where thicker media, or where narrower media (e.g., envelopes, card stock, etc.) are employed. It will be appreciated, for example, that thicker media will tend to bow more significantly when pinched as described above, thus bringing the media closer to the printer""s printhead. In fact, it is possible that the media will actually touch the printhead, causing ink to smear on the media, and potentially damaging the printhead. Correspondingly, narrow media may experience skew due to contact with the wing and/or guide mechanism, particularly where the media is urged into the aforementioned sheet-stiffening bow. In the past, variances in the degree of media bow have been addressed by increasing printhead-to-media spacing, potentially at the expense of print quality and printer size. This, however, has failed to address problems related to media skew.
What is needed is a media handling solution which provides printed sheets with adequate drying time, which addresses problems related to differences in media type (e.g., ink smear) and media skew.
A system is provided for use in a printer, the system including a media width sensor which is configured to sense the presence of media at a predetermined transverse position within the media path. By sensing media at such predetermined transverse position, it is possible to determine media width, and thus to identify media type. Correspondingly, it is possible to adapt media handling to address particular characteristics of the media, namely, thickness of the media and width of the media sheets.
In particular, the invented system employs a media support positioned along the media path downstream from the width sensor and configured to support printed media of a first type upon identification of the media by the media width sensor as being of the first type, and configured to refrain from support of printed media of a second type upon identification of the media by the media width sensor as being of the second type. Typically, it is desirable to support thinner media (such as single-layer A-size paper), but to refrain from supporting thicker media (such as envelopes, card stock or the like). Similarly, it is desirable to support wider types of media (such as A-size paper), but to refrain from supporting narrower media (such as envelopes, card stock or the like).
In one embodiment, the media width sensor takes the form of an optical sensor positioned in a predetermined transverse position along the media path such that media which exceeds a corresponding width is sensed by the sensor so as to identify the media as being of the first type. The sensor may be fixed in such predetermined transverse position, or selectively moved to such position via a reciprocating carriage or the like. It also will be appreciated, that the sensor may take the form of a mechanical sensor or any other sensor capable of detecting presence of media at a predetermined transverse position within the media path.
Alternatively, the media width sensor may take the form of an optical sensor configured to scan the media path. Upon detecting a threshold change in luminance, an edge of the media may be identified, making it possible to determine media width based on a known position of the sensor relative to an edge of the media path. Upon determining the media width, such width may be compared to a table of known media widths in order to determine media type, thus determining media thickness, and whether to support printed media as it exits the output port.
The media support typically takes the form of a pair of pivotal wings configured to selectively urge printed media into engagement with opposing pinch members, thus creating the desired sheet-stiffening bow in the printed sheet. This sheet-stiffening bow is desirable for thin media (e.g. A-size paper) in order to momentarily support the printed media above the output stack and to prevent sheet sail. However, thicker media (e.g. envelopes) tends to bow undesirably when supported by the aforementioned pivotal wings, potentially resulting in contact with the printer""s printhead. This, in turn, may result in smearing of ink on the media, or even damage to the printhead. Furthermore, narrower media (which typically is supported on only one side of the media sheet) may experience media skew, typically due to differing frictional effects on opposite sides of the printed sheet.
Accordingly, the invented system provides a media handling method which includes: 1) identifying a type of media passing through the media path; 2) printing on the media; and 3) where the identified media is of a first type, supporting the media upon printing on the media, and where the media is of a second media type, refraining from support of the media upon printing on the media. As indicated above, the media may be identified either via a sensor in a predetermined transverse position to identify media within a predetermined width range (e.g.  greater than 5-inches), or via an optical sensor which scans the media to determine actual media width. In either event, the determined media width may be matched to a corresponding media type.
In one embodiment, media is considered to be of the first type if it is a thin (e.g. single layer) media such as A-size paper, and is considered to be of the second type if it is thick (e.g. multi-layer) media such as envelopes or the like. Similarly, narrow media (e.g. cards, letter envelopes, etc) may be considered to be of the second type (regardless of media thickness) in order to address problems of media skew.
Furthermore, it may be assumed that the initial media sheet of a print job is of the second type, the printer being directed to retract the wings prior to printing the initial media sheet of the print job. This addresses problems of printhead-to-media spacing and media skew where there is insufficient time to identify media type prior to beginning printing. The initial media sheet of the present print job may be printed at a predetermined elapsed time after completing the preceding print job, thus allowing adequate time for the last sheet of the preceding print job to dry. The actual media type of the initial sheet is determined during printing as the initial sheet passes through the media path.
Upon determining the actual media type of the initial sheet of a present print job, it may be assumed that all subsequent media sheets of the present print job are of the same media type. Subsequent media sheets of the present print job thus may be treated in accordance with the aforementioned methodology. Where the identified media is of a first media type, the wings are extended such that the media is supported upon printing on the media. Where the media is of a second media type, the wings are retracted so as to refrain from support of the media upon printing on the media.